Efficient use of petroleum feedstock requires a refiner to convert relatively high molecular weight hydrocarbons to more valuable lower molecular weight gasoline range hydrocarbon materials. Catalytic cracking is one process used to produce the more valuable gasoline range materials.
Modern catalytic cracking processes typically react hydrocarbon vapors with a hot zeolitic cracking catalyst in a fluidized riser reactor. The cracking reaction occurs as the catalyst and feedstock rise through the riser reactor, with a reaction mixture of predominantly spent catalyst and lower molecular weight hydrocarbons being discharged from the upper end of the reactor. After rising through the reactor, spent catalyst must be separated from the reaction mixture so that the cracked hydrocarbon products can be further processed and so that spent catalyst can be regenerated and reused.
In open vapor path catalytic cracking systems such as those disclosed in U.S. Pat. Nos. 4,390,503, 4,500,423, 4,606,814 and 4,701,307, an initial catalyst disengagement step typically is accomplished by discharging spent catalyst from the upper end of the riser reactor into a volumetrically large disengagement vessel which surrounds the system. In such a system, the momentum of discharged catalyst particles causes the particles to shoot upwardly through a dilute phase fluidized upper region of the vessel and then settle downwardly into a dense phase fluidized lower region of the vessel. A mixture of cracked hydrocarbon vapors and some spent catalyst passes from the dilute phase of the system into one or more cyclone separators, or "cyclones". The cyclones cyclonically remove spent catalyst not removed in the initial disengagement step and discharge a further catalyst-depleted mixture into a generally closed vapor path leading into the reactor outlet plenum and then out of the vessel, with the catalyst collected by each cyclone flowing down to the catalyst dense bed through a cyclone bottom outlet. At the same time, the dense phase bed of accumulating catalyst is stripped of entrained hydrocarbon vapors by passing stripping steam through the bed. This stripping process releases a mixture of stripped vapors and stripping steam, or "stripping gas", into the dilute phase vessel volume located above the dense bed. The stripping gas entering the dilute phase enters the cyclones along with the dilute phase materials already discussed.
Open vapor path systems like those just described provide the advantage of damping pressure and catalyst surges known to occur in catalytic cracking riser reactors. Causes of these surges include normal catalyst flow irregularities, equipment malfunctions, the sudden vaporization of water present in feedstock, and various other unit pressure upsets. Because these riser surges are damped into the volumetrically large disengagement vessel before the reaction products enter the secondary separation equipment, the surges do not propagate through the secondary separation equipment and degrade the separation efficiency of downstream devices as they otherwise would if not damped into the vessel volume.
Unfortunately, the design of open vapor path systems has been found to contribute to the undesired secondary thermal cracking of gasoline range materials when operated in the 1000 degree plus Fahrenheit temperature range common in modern catalytic cracking reactor systems. Because the cracked products mix with the large disengagement vessel volume before being withdrawn from the vessel by the secondary separation equipment, the cracked products can reside in the vessel long enough at high enough temperatures to significantly affect product yield. For example, estimates show that as much as ten percent of the desired gasoline range products can be lost if these products are exposed to temperatures of 1100.degree. F. for as little as 4 to 5 seconds. Furthermore, the presence of cracking catalyst in the dilute phase of the unit can lead to overcracking of hydrocarbon vapors in that region.
To prevent undesired overcracking and secondary thermal cracking, some refiners have turned to closed vapor path systems in which reaction products pass along a closed vapor path from a riser reactor directly to catalyst disengagement equipment. Such closed systems may reduce overcracking because cracked hydrocarbon vapors and spent catalyst are immediately discharged into a cyclone separator, thereby potentially effecting a rapid catalyst disengagement. The system may also reduce thermal cracking because vapors are not discharged into the relatively large disengagement vessel with its associated long gas residence times. One such representative closed vapor path system is that disclosed in Haddad, U.S. Pat. No. 4,502,947.
While the use of closed systems such as Haddad's may minimize undesired thermal cracking and overcracking, closed systems can suffer from an inability to mitigate the effects of pressure and catalyst surges. Specifically, because surges no longer vent into a large disengagement vessel volume, surges propagate through secondary separation equipment such as cyclones, thereby disrupting the motion of materials inside the equipment. This disruption reduces the separation equipment's separation efficiency and can cause substantial quantities of cracking catalyst to propagate downstream of the separation equipment. In some instances, cracking catalyst can propagate beyond the catalyst separation equipment, leading to post-separator cracking and contamination of fractionator feedstreams, thereby impacting process operability.
One potential method for dealing with unwanted surges in closed systems is to employ a mechanical solution such as the surge activated trickle valves disclosed in U.S. Pat. Nos. 4,581,205 and 4,588,558. This method may permit surges to be vented into a large disengagement vessel volume, but is undesirable because it increases the mechanical complexity of the separation equipment and because it requires the continued operation of mechanical devices in the thermally severe and erosive catalytic cracking environment.
Another potential solution to surge and secondary cracking problems is to employ an "open-bottomed" cyclone design as disclosed by Farnsworth in U.S. Pat. No. 4,478,708. In this design, catalytically-cracked products and spent catalyst follow a closed vapor path into a cyclone having a bottom which opens into a relatively large disengagement vessel volume. Catalyst is cyclonically separated in the cyclone in much the same manner as in closed cyclones well known in the art. However, instead of falling into a dipleg, separated catalyst simply falls through the open cyclone bottom into the lower portion of the disengagement vessel for stripping and collection. Catalyst-depleted gas passes from the top of the cyclone through secondary separation cyclones as in many traditional closed-bottomed cyclone systems.
Farnsworth's design seems to succeed because the lower pressure downstream of his open-bottomed cyclone causes the cyclone to appear to be a closed vapor path for gases even though the bottom of the cyclone is open. Only when cyclone inlet pressure increases significantly, such as under surge conditions, does the open bottom appear to offer a vapor path into the large disengagement vessel volume. Thus, Farnsworth's design may represent an improvement over the other designs already discussed.
While Farnsworth's open-bottomed cyclone design may provide a partial solution to the surge and secondary cracking problems inherent in closed-vapor path catalytic cracking operations, his design suffers from a serious disadvantage that stems from the use of the open-bottomed cyclone as the primary solids disengagement device. Specifically, while separated catalyst is falling downwardly toward the open bottom, stripping gas simultaneously must flow up into the cyclone's open bottom. This countercurrent flow of catalyst and vessel vapors in a cyclone can cause separated catalyst to become reentrained in the entering stripping gas, thereby reducing the efficiency of the separator. This problem is particularly acute in heavily-loaded cyclones such as Farnsworth's where the lack of a pre-cyclone disengagement device requires that much of the inventory of cracking catalyst must be discharged through the open bottoms of the cyclones. While improvements to open bottom cyclone systems are disclosed in our commonly assigned U.S. applications having Ser. No. 07/815,281 and 07/815,286, refiners also desire improved non-open bottom cyclone system designs.
What is needed is a generally closed vapor path catalytic cracking reactor system which can reduce undesired thermal cracking, minimize the effects of pressure transients, and which does not require stripping gas to pass countercurrently through large fractions of the circulating cracking catalyst inventory under non-surge conditions.